1. Field of the Invention
The present invention relates to a horizontal linearity correcting circuit suitable for use in an image display apparatus for displaying e.g., a television signal and an image signal from a personal computer.
2. Description of Related Art
For example, a horizontal deflecting circuit as shown in FIG. 1A is basically used in an image display apparatus for displaying, for example, a television signal. Namely, when a transistor Tr is turned on in FIG. 1A, a resonance capacitor Cc is short-circuited and an electric current having a constant inclination from a power source Vcc flows through a horizontal deflecting winding DY. When the transistor Tr is next turned off, the resonance capacitor Cc and the horizontal deflecting winding DY are resonated so that the electric current flowing through the horizontal deflecting winding DY becomes a sine wave.
A diode D is turned on when this electric current reaches a negative maximum value, i.e., when the potential of the horizontal deflecting winding DY across its both ends exceeds a zero potential and is lowered. Thus, the electric current having the constant inclination again flows through the horizontal deflecting winding DY. The diode D is turned off when a collector voltage of the transistor Tr becomes higher than the ground voltage. Accordingly, if the transistor Tr is turned on before the diode D is turned off, the electric current flows through the horizontal deflecting winding in an opposite direction without interruption so that the above operation is repeated. This repeating operation forms one period of horizontal deflection.
However, when a beam is made to scan at a constant speed in one period of such horizontal deflection, images at left-hand and right-hand ends of a tube screen are extended in comparison with a central portion of the tube screen. Therefore, as shown in FIG. 1B, a capacitor Cs is connected in series to the horizontal deflecting winding DY so that the waveform of a deflecting electric current is made dull. Thus, the scanning speed of the beam at each of both left-hand and right-hand ends of a horizontal scanning line is reduced so that left-hand and right-hand extensions of the image are vanished. This is an "S-character correction" and the above capacitor Cs is a capacitor for the S-character correction.
When the scanning operation is performed from a top of the screen to a bottom of the screen, the difference in path of the beam electric current from a deflecting center is geometrically long at the top and bottom of the tube screen so that an image is extended at the top and bottom of the tube screen and is shrunk at the center of the tube screen. This is an image distortion called a pin distortion. Therefore, a voltage of the capacitor Cs for the S-character correction is modulated in a parabola shape to correct such an image distortion such that this voltage is small at the beginning and the end of a vertical period and is large in an intermediate portion of this vertical period. This correction is a "pin distortional correction".
For example, as shown in FIG. 1B, a power source voltage Vcc is modulated in a parabola shape of the vertical period in the actual circuit and this power source Vcc is connected through a winding LOC in parallel to a series circuit of the horizontal deflecting winding DY and the capacitor Cs for the S-character correction. Thus, the power source voltage applied to the capacitor Cs for the S-character correction is modulated in the parabola shape to thereby realize the above-mentioned pin distortional correction. Thus, the above S-character correction and the pin distortional correction can be carried out in the horizontal linearity correcting circuit of the image display apparatus.
However, for example, when such corrections are carried out and the scanning operation is performed from the top of the screen to the bottom of the screen as mentioned above, horizontal deflecting angles at upper and lower ends of the tube screen and an X-axis shown in FIG. 2 are different from each other since the differences in path of the beam electric current from the deflecting center are geometrically different from each other on the tube screen, etc. Therefore, S-character correcting amounts of the deflecting electric current are also different from each other. Namely, a stronger S-character correcting amount is required at the center of the tube screen having a deflecting angle larger than that at each of the upper and lower ends of the tube screen.
Therefore, when the same S-character correcting amount is provided on the entire screen, the S-character correcting amount is small near the center of the screen in comparison with the upper and lower ends of the screen. As shown in FIG. 2, longitudinal lines of intermediate portions between the left-hand and right-hand ends of the screen and a Y-axis draw arcs in a convex shape toward the center even when the pin distortional correction can be made in both left-hand and right-hand end portions of the screen. This image distortion is called an "intermediate pin distortion" and tends to be strongly caused in comparison with a tube screen formed on a spherical surface particularly in a cathode ray tube having a cylindrical tube screen. This distortion also becomes notable when the deflecting angle is increased.
For example, a horizontal linearity correcting circuit generally called a diode modulator system is proposed with respect to such an intermediate pin distortion. For example, as shown in FIG. 3, this horizontal linearity correcting circuit of the diode modulator system is realized by constructing the above circuit of FIG. 1B at two stages and further modulating the capacity of a capacitor Cs2 corresponding to the capacitor for the S-character correction at a lower stage.
Namely, an electric current flowing through this capacitor Cs2 is changed in this circuit by changing voltages across both ends of the capacitor Cs2 at the lower stage so that an apparent capacity value of the capacitor Cs2 can be changed. Thus, resonance frequencies of the capacitors Cs1 and Cs2 for the S-character correction and the horizontal deflecting winding DY1 are changed so that the S-character correcting amount can be changed. Accordingly, for example, the above intermediate pin distortion can be corrected by modulating the power source voltage Vcc in the vertical period in this circuit.
However, the correcting amount of the intermediate pin distortion cannot be necessarily sufficiently obtained in such a circuit of FIG. 3. Namely, capacity values of the two capacitors Cs1 and Cs2 for the S-character correction are set in the circuit of FIG. 3 while trackings of the horizontal linearity and the intermediate pin distortional correction are performed. It is necessary to particularly set the correcting amount of the intermediate pin distortion to be large in a cathode ray tube CRT having a large deflecting angle.
In this case, the capacity value of the capacitor Cs2 for the S-character correction at the lower stage is reduced to increase the correcting amount of the intermediate pin distortion. However, in this case, the S-character correcting amount is also increased and a horizontal amplitude is shrunk at its both ends. Further, the capacity value of the capacitor Cs1 for the S-character correction at the upper stage is increased to reduce this S-character correcting amount. However, in reality, there is a limit in the increase in the capacity value of the capacitor so that a degree of freedom with respect to the reduction in the S-character correcting amount is small.
It is considered as another method that a coil Ls2 at the lower stage is set to a transformer structure and a secondary winding thereof is connected in series to the horizontal deflecting winding DY1 so as to increase the correcting amount and relax the shrinkages at both the ends of the horizontal amplitude. However, problems of a reduction in the variable range of a horizontal deflecting width, a large-sized structure of the transformer, etc. are caused. Further, the capacity values of the two capacitors are respectively fixed so that the correcting amount is changed by dispersion of parts, etc.
For example, when the horizontal deflecting width is changed in the above circuit, there is a fear that the horizontal linearity and the intermediate pin distortion become worse. Namely, when the capacity values of the above two capacitors Cs1, Cs2 for the S-character correction are set such that the horizontal linearity and the intermediate pin distortion are preferable at the time of a certain horizontal amplitude on the tube screen, the S-character correction is excessively made in the case of an amplitude equal to or smaller than this horizontal amplitude and is insufficiently made in the case of an amplitude equal to or greater than this horizontal amplitude. Therefore, for example, when the horizontal amplitude is shrunk to display a picture image of 4 to 3 on the tube screen of 16 to 9, the picture image is shrunk at its both ends and is distorted in a barrel shape in its intermediate portion.
With respect to problems to be solved, the correcting amount of the intermediate pin distortion is not necessarily obtained sufficiently in the conventional circuit. Further, when the horizontal deflecting width is changed, there is a fear that the horizontal linearity, the intermediate pin distortion, etc. become worse.